The income elasticity of demand is positive for
normal goods and negative for inferior goods.

The income elasticity of demand is positive for normal goods
means the relationship between income and demand for good is
positive, which further means as the incomes rises the demand fort
he goods also increases. For instance, the food consumption, the
purchase of clothes, the purchase of cars and many other increases
as the income rises. Thus, these goods are termed as normal
goods.

On the other hand, the negative income elasticity of demand
means the relationship between income and demand for goods is
negative, means as the income raises the demand for the goods does
not rises or even decreases. For instance, the used cars (second
hand cars), if there is an increase in income people will not buy
second hand car, instead they will book a new one. Therefore these
goods are termed as inferior goods.